1. Field of Invention
The present invention relates to a system and method for reducing rotation noise in a magnetoelastic torque sensing device. More specifically, the present invention relates to a system and method for creating one or more magnetically conditioned regions on a rotatable shaft or disk-shaped torque sensing element, wherein rotation noise produced by the element due to magnetic field variations is substantially negated.
2. Description of the Related Art
In the control of systems having rotating drive shafts, torque and speed are fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable, and inexpensive manner has long been a primary objective of such control system designs.
U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis, the entireties of which are incorporated by reference herein, describe torque sensing devices that include a shaft, and a ring of magnetoelastically active material disposed around the shaft. U.S. Pat. No. 6,047,605 to Garshelis, the entirety of which is incorporated by reference herein, describes a torque sensing device that does not include a ring, rather, the shaft itself is formed of material that includes a magnetoelastically active region. In each of those torque sensing devices, the magnetoelastically active region includes one or more magnetically conditioned regions that are magnetically polarized in a circumferential direction and possess sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the shaft, to the circumferential direction when the applied torque is reduced to zero. When a torque is applied to the shaft, the circumferential magnetic orientations of the magnetically conditioned regions reorient such that they exhibit helical magnetic orientations having both circumferential and axial components. Magnetic field sensors mounted proximate to the magnetically conditioned regions, without contacting the regions, are configured to sense only the axial components of magnetic fields produced by the regions.
Theoretically, when no torque is applied to the shaft of the '555, '059, or '605 patents, no axial magnetic field components are present, and the magnetic field sensors produce no output. When torque is applied to the shaft, axial magnetic field components are produced in proportion to the applied torque, and the magnetic field sensors output electrical signals that are indicative of the applied torque.
U.S. Pat. No. 6,513,395 to Jones, the entirety of which is incorporated by reference herein, describes a torque sensing device that includes a disk-shaped member having a magnetoelastically active region disposed thereon. The magnetoelastically active region includes one or more magnetically conditioned regions that are magnetically polarized in a circumferential direction when no torque is applied to the disk. When a torque is applied to the disk, the circumferential magnetic orientations of the magnetically conditioned regions reorient such that they exhibit circumferential, axial, and radial components. Magnetic field sensors mounted proximate to the magnetically conditioned regions, without contacting the regions, are configured to sense only the axial, or only the radial, components of magnetic fields produced by the regions.
Theoretically, when no torque is applied to the disk of the '395 patent, no axial or radial magnetic field components are present, and the magnetic field sensors produce no output. When torque is applied to the disk, axial and radial magnetic field components are produced in proportion to the applied torque, and the magnetic field sensors output electrical signals that are indicative of the applied torque.
U.S. patent application Ser. No. 13/368,079 to Lee, the entirety of which is incorporated by reference herein, describes a torque sensing device with a disk-shaped member having a magnetoelastically active region that includes magnetically conditioned regions that are magnetically polarized in the axial direction. Magnetic field sensors oriented in a circumferential direction output electrical signals that are indicative of a torque applied to the disk.
Each of the aforementioned torque sensing devices generally incorporates a torque transducer (e.g., ring, shaft, or disk) having a magnetoelastically active region that is formed by first providing a material that possesses sufficient anisotropy to return the magnetization to the quiescent, or initial direction, when the applied torque is reduced to zero. Magnetic anisotropy may be induced by physical working of the transducer or by other methods. Illustrative methods for inducing magnetic anisotropy are disclosed in the '555 and '059 patents.
Following the introduction of magnetic anisotropy to the transducer, the transducer must be polarized in the desired direction or directions (i.e., one or more magnetically conditioned regions must be formed in the magnetoelastically active region). The '555 and '059 patents describe a method of polarizing a transducer by rotating it in the field near two opposite magnetic poles as provided by a horseshoe magnet. The '079 application describes a method of polarizing a transducer by rotating it in the field near a rectangular NdFeB magnet.
During operation, each of the aforementioned torque sensing devices generally incorporates a torque transducer with a magnetically conditioned region that is magnetically polarized in a first direction when the transducer is in the quiescent state (i.e., when no torque is applied to the transducer). One or more magnetic field sensors are mounted proximate to the magnetically conditioned region such that each magnetic field sensor has a sensitive direction that is perpendicular to the first direction. The torque sensing devices rely on the principle that, because the magnetic field sensors are not capable of sensing magnetic field components that are perpendicular to their sensitive directions, each magnetic field sensor produces no output when the transducer is in the quiescent state. When a torque is applied to the transducer, the magnetic field produced by the magnetically conditioned region reorients such that magnetic field components are sensible by the magnetic field sensors, and the magnetic field sensors output electrical signals that are indicative of the applied torque.
Theoretically, the methods used to magnetize (i.e., polarize) the transducers of the prior art torque sensing devices result in consistent dipole alignment in accordance with a magnetic field applied in a particular direction and at a constant strength. However, those prior art transducers often have physical irregularities that may result from inconsistent alloy composition or crystal displacements, or may be otherwise unintentionally introduced during the fabrication process. Such irregularities can result in localized areas within the magnetoelastically active region in which the direction and/or strength of the magnetic field varies from its theoretical value. For example, in theory, a circumferentially polarized region exhibits no magnetic field components in the radial or axial directions while in the quiescent state. However, in actuality, such a region will exhibit radial and/or axial magnetic field components while in the quiescent state due to the aforementioned magnetic field variations. Such magnetic field variations are referred to as rotation noise, and can negatively affect the accuracy of prior art torque sensing devices.
Therefore, there exists a need for a method of magnetizing a torque transducer, wherein the method accounts for physical irregularities in the transducer, and wherein the method results in the elimination of rotation noise when the transducer is used as a torque sensing device. There further exists a need for a system for performing such a method.